COLLEGE PHYSICS

Chapter 23 ELECTROMAGNETIC INDUCTION, AC CIRCUITS, AND

ELECTRICAL TECHNOLOGIESPowerPoint Image Slideshow

FIGURE 23.1

This wind turbine in the Thames Estuary in the UK is an example of induction at work.

Wind pushes the blades of the turbine, spinning a shaft attached to magnets. The

magnets spin around a conductive coil, inducing an electric current in the coil, and

eventually feeding the electrical grid. (credit: phault, Flickr)

FIGURE 23.2

Physics, like this butterfly, has inherent symmetries. (credit: Thomas Bresson)

FIGURE 23.3

Faraday’s apparatus for demonstrating that a magnetic field can produce a current. A change

in the field produced by the top coil induces an emf and, hence, a current in the bottom coil.

When the switch is opened and closed, the galvanometer registers currents in opposite

directions. No current flows through the galvanometer when the switch remains closed or

open.

FIGURE 23.4

Movement of a magnet relative to a coil produces emfs as shown. The same emfs are

produced if the coil is moved relative to the magnet. The greater the speed, the greater

the magnitude of the emf, and the emf is zero when there is no motion.

FIGURE 23.5

Rotation of a coil in a magnetic field

produces an emf. This is the basic

construction of a generator, where work

done to turn the coil is converted to

electric energy. Note the generator is

very similar in construction to a motor.

FIGURE 23.6

Magnetic flux Φ is related to the magnetic field and the area over which it exists. The

flux Φ = 𝐵𝐴 cos θ is related to induction; any change in Φ induces an emf.

FIGURE 23.7

(a) When this bar magnet is thrust into the coil, the strength of the magnetic field increases in the coil. The

current induced in the coil creates another field, in the opposite direction of the bar magnet’s to oppose the

increase. This is one aspect of Lenz’s law—induction opposes any change in flux. (b) and (c) are two other

situations. Verify for yourself that the direction of the induced Bcoil shown indeed opposes the change in flux

and that the current direction shown is consistent with RHR-2.

FIGURE 23.8

Recording and playback heads used with audio and video magnetic tapes. (credit:

Steve Jurvetson)

FIGURE 23.9

Electromagnetic induction used in transmitting electric currents across mediums. The

device on the baby’s head induces an electrical current in a receiver secured in the

bone beneath the skin. (credit: Bjorn Knetsch)

FIGURE 23.11

(a) A motional emf = Bℓv is induced between

the rails when this rod moves to the right in

the uniform magnetic field. The magnetic

field B is into the page, perpendicular to the

moving rod and rails and, hence, to the

area enclosed by them. (b) Lenz’s law gives

the directions of the induced field and

current, and the polarity of the induced emf.

Since the flux is increasing, the induced

field is in the opposite direction, or out of

the page. RHR-2 gives the current direction

shown, and the polarity of the rod will drive

such a current. RHR-1 also indicates the

same polarity for the rod. (Note that the

script E symbol used in the equivalent

circuit at the bottom of part (b) represents

emf.)

FIGURE 23.12

Motional emf as electrical power

conversion for the space shuttle is the

motivation for the Tethered Satellite

experiment. A 5 kV emf was predicted to

be induced in the 20 km long tether while

moving at orbital speed in the Earth’s

magnetic field. The circuit is completed

by a return path through the stationary

ionosphere.

FIGURE 23.13

A common physics demonstration device for exploring eddy currents and magnetic damping.

(a) The motion of a metal pendulum bob swinging between the poles of a magnet is quickly damped by the action of eddy

currents.

(b) There is little effect on the motion of a slotted metal bob, implying that eddy currents are made less effective.

(c) There is also no magnetic damping on a nonconducting bob, since the eddy currents are extremely small.

FIGURE 20.14

A more detailed look at the conducting plate passing between the poles of a magnet.

As it enters and leaves the field, the change in flux produces an eddy current. Magnetic

force on the current loop opposes the motion. There is no current and no magnetic drag

when the plate is completely inside the uniform field.

FIGURE 23.15

Eddy currents induced in a slotted metal plate entering a magnetic field form small

loops, and the forces on them tend to cancel, thereby making magnetic drag almost

zero.

FIGURE 23.16

Magnetic damping of this sensitive

balance slows its oscillations. Since

Faraday’s law of induction gives the

greatest effect for the most rapid change,

damping is greatest for large oscillations

and goes to zero as the motion stops.

FIGURE 23.17

Metals can be separated from other trash by magnetic drag. Eddy currents and

magnetic drag are created in the metals sent down this ramp by the powerful magnet

beneath it. Nonmetals move on.

FIGURE 23.18

A soldier in Iraq uses a metal detector to search for explosives and weapons. (credit:

U.S. Army)

FIGURE 23.19

The rows of rare earth magnets (protruding horizontally) are used for magnetic braking

in roller coasters. (credit: Stefan Scheer, Wikimedia Commons)

FIGURE 23.20

When this generator coil is rotated

through one-fourth of a revolution, the

magnetic flux Φ changes from its

maximum to zero, inducing an emf.

FIGURE 23.21

A generator with a single rectangular coil rotated at constant angular velocity in a

uniform magnetic field produces an emf that varies sinusoidally in time. Note the

generator is similar to a motor, except the shaft is rotated to produce a current rather

than the other way around.

FIGURE 23.22

The emf of a generator is sent to a light bulb with the system of rings and brushes

shown. The graph gives the emf of the generator as a function of time. Emf0 is the peak

emf. The period is 𝑇 = 1 / 𝑓 = 2𝜋 / 𝜔 , where 𝑓 is the frequency. Note that the script E

stands for emf.

FIGURE 23.23

Split rings, called commutators, produce

a pulsed DC emf output in this

configuration.

FIGURE 23.24

Steam turbine/generator. The steam produced by burning coal impacts the turbine

blades, turning the shaft which is connected to the generator. (credit: Nabonaco,

Wikimedia Commons).

FIGURE 23.25

The coil of a DC motor is represented as a resistor in this schematic. The back emf is

represented as a variable emf that opposes the one driving the motor. Back emf is zero

when the motor is not turning, and it increases proportionally to the motor’s angular

velocity.

FIGURE 23.26

The plug-in transformer has become increasingly familiar with the proliferation of

electronic devices that operate on voltages other than common 120 V AC. Most are in

the 3 to 12 V range. (credit: Shop Xtreme)

FIGURE 23.27

Transformers change voltages at several points in a power distribution system. Electric power is

usually generated at greater than 10 kV, and transmitted long distances at voltages over 200 kV—

sometimes as great as 700 kV—to limit energy losses. Local power distribution to neighborhoods or

industries goes through a substation and is sent short distances at voltages ranging from 5 to 13 kV.

This is reduced to 120, 240, or 480 V for safety at the individual user site..

FIGURE 23.28

A typical construction of a simple

transformer has two coils wound on a

ferromagnetic core that is laminated to

minimize eddy currents. The magnetic

field created by the primary is mostly

confined to and increased by the core,

which transmits it to the secondary coil.

Any change in current in the primary

induces a current in the secondary.

FIGURE 23.29

Transformers do not work for pure DC

voltage input, but if it is switched on and

off as on the top graph, the output will

look something like that on the bottom

graph. This is not the sinusoidal AC most

AC appliances need.

FIGURE 23.31

Schematic of a simple AC circuit with a voltage source and a single appliance

represented by the resistance 𝑅. There are no safety features in this circuit.

FIGURE 23.32

The three-wire system connects the neutral wire to the earth at the voltage source and user location,

forcing it to be at zero volts and supplying an alternative return path for the current through the earth. Also

grounded to zero volts is the case of the appliance. A circuit breaker or fuse protects against thermal

overload and is in series on the active (live/hot) wire. Note that wire insulation colors vary with region and it

is essential to check locally to determine which color codes are in use (and even if they were followed in the

particular installation).

FIGURE 23.33

The standard three-prong plug can only be inserted in one way, to assure proper

function of the three-wire system.

FIGURE 23.34

Worn insulation allows the live/hot wire to

come into direct contact with the metal

case of this appliance.

(a) The earth/ground connection being

broken, the person is severely

shocked. The appliance may operate

normally in this situation.

(b) With a proper earth/ground, the

circuit breaker trips, forcing repair of

the appliance.

FIGURE 23.35

AC currents can induce an emf on the case of an appliance. The voltage can be large

enough to cause a shock. If the case is grounded, the induced emf is kept near zero.

FIGURE 23.36

A ground fault interrupter (GFI) compares the currents in the live/hot and neutral wires

and will trip if their difference exceeds a safe value. The leakage current here follows a

hazardous path that could have been prevented by an intact earth/ground wire.

FIGURE 23.37

GFI compares currents by using both to induce an emf in the same coil. If the currents

are equal, they will induce equal but opposite emfs.

FIGURE 23.38

An isolation transformer puts a large resistance between the original voltage source

and the device, preventing a complete circuit between them.

FIGURE 23.39

These coils can induce emfs in one another like an inefficient transformer. Their mutual

inductance M indicates the effectiveness of the coupling between them. Here a change

in current in coil 1 is seen to induce an emf in coil 2. (Note that " E2 induced" represents

the induced emf in coil 2.)

FIGURE 23.40

The heating coils of an electric clothes dryer can be counter-wound so that their

magnetic fields cancel one another, greatly reducing the mutual inductance with the

case of the dryer.

FIGURE 23.41

FIGURE 23.42

Through rapid switching of an inductor, 1.5 V batteries can be used to induce emfs of

several thousand volts. This voltage can be used to store charge in a capacitor for later

use, such as in a camera flash attachment.

FIGURE 23.43

The familiar security gate at an airport can not only detect metals but also indicate their

approximate height above the floor. (credit: Alexbuirds, Wikimedia Commons)

FIGURE 23.44

(a) An RL circuit with a switch to turn current on and off. When in position 1, the battery, resistor, and inductor are in series

and a current is established. In position 2, the battery is removed and the current eventually stops because of energy loss

in the resistor.

(b) A graph of current growth versus time when the switch is moved to position 1.

(c) A graph of current decay when the switch is moved to position 2.

FIGURE 23.45

(a) An AC voltage source in series with an inductor having negligible resistance.

(b) Graph of current and voltage across the inductor as functions of time.

FIGURE 23.46

(a) An AC voltage source in series with a capacitor C having negligible resistance.

(b) Graph of current and voltage across the capacitor as functions of time.

FIGURE 23.47

(a) An AC voltage source in series with a resistor.

(b) Graph of current and voltage across the resistor as functions of time, showing them

to be exactly in phase.

FIGURE 23.48

An RLC series circuit with an AC voltage source.

FIGURE 23.49

This graph shows the relationships of the voltages in an RLC circuit to the current. The

voltages across the circuit elements add to equal the voltage of the source, which is

seen to be out of phase with the current.

FIGURE 23.50

A graph of current versus frequency for two RLC series circuits differing only in the

amount of resistance. Both have a resonance at 𝑓0 , but that for the higher resistance is

lower and broader. The driving AC voltage source has a fixed amplitude 𝑉0 .

FIGURE 23.51

The forced but damped motion of the wheel on the car spring is analogous to an RLC

series AC circuit. The shock absorber damps the motion and dissipates energy,

analogous to the resistance in an RLC circuit. The mass and spring determine the

resonant frequency.

FIGURE 23.52

An LC circuit is analogous to a mass

oscillating on a spring with no friction and

no driving force. Energy moves back and

forth between the inductor and capacitor,

just as it moves from kinetic to potential

in the mass-spring system.

FIGURE 23.54

A circular coil of wire is stretched in a magnetic field.

FIGURE 23.55

Capacitors and inductors. Capacitor with high frequency and low frequency.

FIGURE 23.56

(a) The planes of the two coils are perpendicular.

(b) The wire is perpendicular to the plane of the coil.

FIGURE 23.57

(a) The coils lie in the same plane.

(b) The wire is in the plane of the coil

FIGURE 23.58

Repeat the previous problem with the battery reversed.

FIGURE 23.59

A coil is moved into and out of a region of uniform magnetic field.

FIGURE 23.60

A person can be shocked even when the case of an appliance is grounded. The large

short circuit current produces a voltage on the case of the appliance, since the

resistance of the earth/ground wire is not zero.

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